1. Field of the Invention
The present invention relates to binary baseband transmission systems. More particularly, this invention pertains to a method for establishing initial synchronization with a training sequence (preamble) prior to transmission of useful data and for matching a receive filter of a digital receiver in a binary baseband transmission system.
2. Description of the Prior Art
The use of signal-matched filters in a receiver is well known in digital data transmission. Such so-called matched-filter receivers, a simplified example of which is illustrated in the block diagram of FIG. 1, include a receive filter that is matched to the received signal as receive filter. The signal is sampled at the character clock rate at the filter's output. The samples are employed by a signal decision circuit to perform the signal decision, possibly following an additional equalization. The use of a matched filter as a receive filter maximizes the signal/noise ratio at the input of the signal decision circuit with correctly timed sampling and with superposed white noise.
In the event that the signal possesses pulse interferences at the output of the matched filter, a signal decision circuit which incorporates the intersymbol interferences into the decision can be used for implementing an optimum receiver. See, for example, G. D. Forney, Jr., "Maximum-Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference," IEEE Transactions on Information Theory, Vol. IT-18, (May 1972) pages 363-378, and J. Huber, "Detektoren und Optimalfilter fur Digital-signals mit Impulsinterferenzen (Detectors and Optimal Filters For Digital Signals With Interpulse Interference)", Frequenz 41 (1987), pages 161-167 and 189-196.
Since the signal decision circuits are technically highly complex, a sub-optimum combination of equalizer and threshold-value decision circuit is employed, as a rule, when pulse interferences occur. Signal equalization reduces the signal/noise ratio at the input of the threshold-value decision circuit, leading to a loss in optimum performance.
If the transmission channel is unknown and/or variable with time, the receive filter and the signal decision circuit or the equalizer must be matched to the received signal.
An optimum receiver in which the receive filter is matched to the received signal by minimizing the noise component of the samples at the output of the receive filter has been proposed. See G. Ungerboeck, "Adaptive Maximum-Likelihood Receiver for Carrier-Modulated Data-Transmission System, " IEEE Transactions on Communications, Vol. COM-22, (May 1974) pages 624-636. Sussman and Monsen have proposed a receiver in which a constructive adding together of the received pulses is achieved by feeding back the delayed received signal. S. M. Sussman, "A Matched Filter Communication System for Multipath Channels," IRE Transactions on Information Theory, Vol. IT-6, (June 1960) pages 367-373 and P. Monsen, "Fading Channel Communications," IEEE Communications Magazine (January 1980), pages 16-25. A matched filter can then be implemented by correlating the received pulse thus determined with the received signal.
Receivers with adaptive equalizers are either implemented with fixed receive filters and "T-spaced equalizer" or as "fractionally spaced equalizer" (FSE). Receivers with fixed receive filter and adaptive T-spaced equalizer are sensitive to sampling clock deviations from the ideal sampling time. An FSE represents the synthesis of a digitally implemented matched filter with a conventional T-spaced equalizer (compare FIG. 2). The FSE is advantageously insensitive to deviations of signal element timing from the ideal sampling time. Compare G. Ungerboeck, "Fractional Tap-Spacing Equalizer and Consequences for Clock Recovery in Data Modems," IEEE Transactions on Communications, Vol. COM-24, No. 8, (August 1976). A summary of the capabilities of adaptive equalizers is provided by S.U.H. Quereshi, "Adaptive Equalization," Proceedings of the IEEE, Vol. 73, No. 9 (September 1985).
The operation of a matched-filter receiver requires correct timing of the sampling of the signal at the output of the matched filter. To insure this, the correct sampling time must be obtained from the received signal. Two operating states can be distinguished in character synchronization. These are, firstly, the state at the beginning of a data transmission in which there is no synchronization and, secondly, the state after synchronization has been achieved and only small fluctuations must be compensated. Without synchronization, no useful data can be received since the error rate of the decision circuit is too high. For this reason, the character synchronization is generally adjusted during the reception of a previously mentioned training sequence. Any equalizer which may be present can only be adjusted after character synchronization has been achieved which, in turn, is accomplished with the aid of a training sequence. See CCITT Recommendation, V. 27bis and V.27ter. The exception is the FSE in which the character synchronization occurs automatically during equalizer adjustment.